Motorized personal mobility devices, such as wheelchairs, are generally well known. The wheels of such devices are typically powered by gearboxes connected thereto. Due to the common uses of a wheelchair, the gearmotor assembly must develop sufficient torque to drive the wheelchair and user through all typical passageways, which includes driving the wheelchair over common obstacles, such as small curbs and doorstops, which most people step over. Accordingly, a large, heavy gearbox is typically required in order to accommodate the large gearing necessary to accomplish this. However, due to the small size and common need for portability of such mobility devices, it is generally advantageous to employ gearboxes that do not require such larger, heavier gearing and gearboxes in order to handle the torque necessary to drive the wheelchair.
In order to accommodate these space and weight considerations, particular gearing arrangements have been suggested to limit the size of the gearboxes used to power the wheels of the chair. For example, in order to provide the necessary power to drive the wheels with gearing that can be housed within this limited space, it has been suggested to employ worming, such as in the design disclosed in U.S. Pat. No. 6,029,763 to Swisher. In this type of arrangement, a worm is used to drive a gear rotatable about an axis of rotation perpendicular to that of the worm, such that the threads of the worm engage the teeth of the gear driven thereby.
Such designs utilize conventional worming (illustrated in FIGS. 6A-B). In this type of gear set, the driven gear (i.e., “worm wheel”) is curved—transverse to the longitudinal, rotational axis of the worm—such that it corresponds to the rounded surface of the worm, thereby providing better contact between the worm and the driven gear. However, with this type of gear set, the driven gear still curves away from the worm along the worm's longitudinal axis, and thus, only one to two gear teeth are in contact with the worm threads at any given time.
Alternative designs for worms, other than that incorporated into modern wheelchair gearmotors, have been suggested. One such design is the use of globoid worm, such as that disclosed in U.S. Pat. No. 4,047,449 to Popov. In this design, the worm is curved in the direction of the longitudinal axis of the worm, transverse to the rotational axis of the driven gear, such that it corresponds to the rounded surface of the driven gear. This type of design also provides improved contact between the worm and the driven gear.
A specific challenge that has remained regarding the gearing for wheelchairs is based on the fact that wheelchairs require the ability to move backwards. This has traditionally been accomplished in one of two ways. Some wheelchairs have a back-drivable feature, where a force is exerted in a backward direction in order to drive the wheel in this direction. In these cases, in order to allow for a low push-back force, the worm must be increased in size. In other wheelchairs, a gearbox neutral feature is used, where a lever is used to separate the gears, thereby leaving the wheels free to roll. In order to employ this type of mechanical release, smaller worming must be employed. Accordingly, because the size objectives of these approaches are diametrically opposed, until now, a single gearbox that is capable of accommodating both the mechanical release and a worm capable of back-drivability has not been feasible.
What is desired, therefore, is a gear assembly for a personal mobility device that maximizes torque output. What is further desired is gear assembly for a personal mobility device that minimizes the necessary size of the gearbox.